1. Field of the Invention
The present invention relates to active pixel sensors. More particularly, the present invention relates to active pixel sensors having improved dynamic range and low leakage.
2. The Prior Art
Active pixel image sensors are known in the art. Multi-color active pixel image sensors are also known, as exemplified by the three-color active pixel sensor disclosed in U.S. Pat. No. 5,965,875 to Merrill.
Dynamic range is a very high profile specification for image sensors, because it affects exposure latitude and the ability to capture a range of light levels close to the range exhibited by human perception. The standard definition of dynamic range is the maximum signal range (signal output at saturation) of a sensor divided by the noise standard deviation of the sensor in darkness. One way to increase the dynamic range of an active pixel sensor is to increase capacitance of the pixel. Increased capacitance at detection and sensing nodes of a pixel results in a greater total signal charge capture capability, thereby increasing the maximum signal range of the pixel. However, the increased dynamic range of the pixel comes at the expense of reduced sensitivity as well as reduced signal-to-noise ratio at low-light-level signals. The sensitivity of the pixel is reduced because more photons are required to give an equivalent variation in voltage at the pixel, and noise increases because the larger pixel area is susceptible to more leakage current which is both an area and perimeter dependent phenomenon, and because a higher capacitance causes a greater charge domain component of KTC noise, sometimes referred to as reset noise.
An additional way to increase the dynamic range is to reduce the noise level. The primary difficulty with this approach is that many of the noise mechanisms are based on fundamental physical principles and behaviors that cannot be easily altered. Once the sensor designer hits the limits of noise reduction in terms of physics, this approach is no longer useful for dynamic range enhancement.
A third way of increasing the dynamic range is to increase the signal range without changing the pixel capacitance. This can be done by increasing the operating voltage of the pixel and therefore directly increasing the total amount of charge that can be collected. Increasing the voltage can introduce some problems, namely increased noise due to leakage current and potential device breakdown or damage due to high electric fields present across thin gate oxide layers.
There are numerous examples of prior-art approaches to increase dynamic range in pixel sensors. U.S. Pat. No. 6,175,383 teaches a pixel design and operation method that permits increased dynamic range. Dynamic range increase is obtained by controlling the reset signal to each pixel individually. This permits different pixels to integrate light for different lengths of time. Pixels that are subject to lower illumination levels can integrate for a long time, building up a substantial signal, and pixels that are exposed to high levels of illumination can integrate for a much shorter period, preventing saturation.
U.S. Pat. No. 5,892,541 teaches a pixel design and operation method for an active pixel sensor that permits the value of each pixel to be read mutliple times during exposure, thereby allowing the progress during integration to be monitored. Pixels that approach saturation prematurely can be reset, and their outputs stored over time. At the end of the total integration period, all of the stored values can be assembled to form an image that is of higher dynamic range than that which is possible without this method.
U.S. Pat. No. 6,011,251 teaches a pixel design and method that permits multiple samplings of the output of the pixel during the integration period. This multi-sampling approach permits the response characteristic of the pixel to be a multiple slope characteristic, with higher sensitivity in dark areas and lower sensitivity in bright areas.
In all three of the above identified patents, greater dynamic range is achieved at the expense of circuit complexity and a certain amount of post-processing of the image data to reconstruct the image.
U.S. Pat. No. 5,614,744 teaches a pixel design that eliminates the active area edge from the photo sensitive node of the pixel, thereby reducing leakage current and therefore permitting larger dynamic range. This method does not completely address the voltage swing that can be achieved with the pixel because the maximum voltage swing is still limited by the magnitude of the electric field that can be tolerated by the gate oxide of the transistors in the pixel. Because the voltage swing is limited to a small range the dynamic range improvement achieved with this invention is smaller than optimal.
U.S. Pat. No. 6,204,524 teaches adding a capacitor in parallel with the photo sensitive node to increase signal to noise ratio and increase the amount of photo-generated charge that can be collected by the pixel. This patent effectively teaches a method of reducing the voltage swing per unit photo-generated charge, which increases the dynamic range, but also decreases the sensitivity.
U.S. Pat. No. 6,160,282, teaches a pixel design that eliminates the presence of silicides in the sensitive nodes, thereby reducing leakage current. Noise reduction and increased dynamic range are achieved by reducing leakage current, but the limitation on voltage range is still present in this invention.